Currently in Japan, almost all secondary batteries mounted on portable electronic devices such as cell phones and laptop computers are lithium secondary batteries. It is predicted that the lithium secondary batteries will be also put in practical use as large-size batteries for hybrid cars, electric power load leveling systems and the like in the future, and their importance becomes increasingly high.
Any of the lithium secondary batteries has, as major constituents, a positive electrode and a negative electrode capable of reversibly occluding and releasing lithium, and further a separator containing a nonaqueous electrolyte solution, or a solid electrolyte.
Among these constituents, electrode active materials under investigation include oxides such as a lithium cobalt oxide (LiCoO2), a lithium manganese oxide (LiMn2O4) and a lithium titanate (Li4Ti5O12), metals such as metallic lithium, lithium alloys and tin alloys, and carbon materials such as graphite and MCMB (mesocarbon microbeads).
The voltage of a battery is determined by difference in the chemical potential depending on the lithium content in each active material. It is a feature of lithium secondary batteries excellent in the energy density that particular combinations of active materials can produce high potential differences.
In particular, the combination of a lithium cobalt oxide LiCoO2 active material and a carbon material as an electrode is widely used in current lithium batteries, because a voltage of nearly 4 V is possible; the charge and discharge capacity (an amount of lithium extracted from and inserted in the electrode) is large; and the safety is high in addition, this combination of the electrode materials is widely used in current lithium batteries.
On the other hand, it has become clear that a lithium secondary batteries with excellent performance in the charge and discharge cycle over a long period is possible in the combination of a spinel-type lithium manganese oxide (LiMn2O4) active material and a spinel-type lithium titanium oxide (Li4Ti5O12) active material as electrode, because the materials make the insertion and extraction reaction of lithium to be smoothly carried out and make a change in the crystal lattice volume accompanying the reaction to be smaller, and the combination is put in practical use.
With respect to chemical batteries such as lithium secondary batteries and capacitors, there are demanded electrode active materials of further high performance (large capacity) in combinations of oxide active materials as described above, because it is predicted that there hereafter become necessary large-size and long-life chemical batteries such as power sources for automobiles, large-capacity backup power sources and emergency power sources.
Titanium oxide-based active materials, in the case where a lithium metal is used as a counter electrode, generate a voltage of about 1 to 2 V. Hence, the possibility of titanium oxide-based active materials with various crystal structures is studied as negative electrode active materials.
There are paid attention to, as electrode materials, active materials such as a spinel-type lithium titanium oxide Li4Ti5O12, a titanium dioxide with sodium bronze-type crystal structure (in the present description, the “titanium dioxide with sodium bronze-type crystal structure” is abbreviated to “TiO2(B)”) and H2Ti12O25 being a titanium oxide containing a hydrogen element in its crystal structure (Patent Literatures 1 to 6, Non Patent Literatures 1 to 5).
These active materials are mainly obtained by firing a starting raw material obtained by mechanically mixing titanium oxide as a Ti raw material and a solid of an alkali metal salt, and followed by an acid treatment and the like (Patent Literatures 1,2 and 4 to 6, Non Patent Literatures 1 to 5).
In the mixing methods using solid samples, however, the mixing state of the samples in the mixing stage is nonhomogeneous in the micro level; the progress of the solid reaction brings the heterogeneity near to homogeneity, but there is a fear that unreacted raw materials remain. Hence, depending on the particle sizes of sample raw materials, firing has to be carried out longer than needed in some cases, and after the firing, crushing and mixing have to be carried out to enhance the homogeneity of the product in some cases.
Further, there is also a case where titanium oxide and an alkali metal salt are mixed and dissolved in water to make a slurry, which is dried by spray drying using a spray drier or spray pyrolysis to thereby prepare a mixed raw material (Patent Literature 3).
In the case where a slurry in which titanium oxide and an alkali metal salt are dissolved in water is dried by spray drying or the like, although the homogeneity of sprayed droplets themselves is held, the alkali metal salt is segregated in the drying process and the mixing state of the titanium oxide and the alkali metal salt is heterogeneous in the micro level, therefore making the product by firing to be nonhomogeneous.